Constant dwell ignition system

ABSTRACT

An electronic triggering circuit for an ignition system to develop electric signals which correspond to the closing and opening of breaker points operates to supply a constant dwell (constant ratio of on-to-off) pulses to the ignition primary winding of the ignition coil, except at low engine speeds whereupon the triggering circuit reverts to a constant &#39;&#39;&#39;&#39;off&#39;&#39;&#39;&#39; time circuit.

United States Patent [1 Weber Mar. 18, 1975 1 CONSTANT DWELL IGNITION SYSTEM [75] lnventor: Howard F. Weber, Scottsdale, Ariz.

[73] Assignee: Motorola, Inc., Franklin Park, 111.

[22] Filed: Nov. 20, 1972 [2]] Appl. No.: 308,125

Primary ExaminerCharles .1. Myhre Assistant Examiner-Ronald B. Cox Attorney, Agent, or FirmMueller, Aichele & Ptak An electronic triggering circuit for an ignition system to develop electric signals which correspond to the closing and opening of breaker points operates to supply a constant dwell (constant ratio of on-to-off) pulses to the ignition primary winding of the ignition coil, except at low engine speeds whereupon the triggering circuit reverts to a constant off time circuit.

[52] US. Cl 123/148 E, 123/117 R [51] Int. Cl. F02p l/00, F02p 5/04 [57] ABSTRACT [58] Field of Search 123/148 E, 117 R [56] References Cited UNITED STATES PATENTS 3,253,163 5/1966 Konopa 123/148 E 3,357,416 12/1967 Huntzinger 123/148 E 3,473,061 /1969 Sauvignet 123/148 E 3,559,629 2/1971 LeMastersu. 123/148 E 3,587,551 6/1971 Harrow 123/148 E 4 Claims, 1 Drawing Figure %,2a 49 47' j z 39 I9 22 24 1 "-36 PATENTED 1 81975v QM V 1 CONSTANT DWELL IGNITION SYSTEM BACKGROUND OF THE INVENTION The Kettering ignition system currently used in vehicles depends upon the energy storage in the primary of 5 a high turns ratio ignition coil to develop the necessary output voltage to fire the spark plug. This energy level is dependent upon the coil current flowing at the time the coil circuit is interrupted by the breaker points to deliver output spark voltage. The coil current that can be reached during the available time is dependent upon coil primary inductance, primary resistance and voltage.

Variable dwell transistorized ignition circuits operating on the Kettering principle have been proposed in which the current through the primary winding of the ignition coil is turned on only shortly before the ignition point and is turned off at the moment the ignition pulse is desired. At low engine speeds, that is when the pickup pulse source for triggering the ignition system operates at a relatively low frequency the current is connected to the ignition coil long before the ignition time and a strong ignition pulse is provided. At higher speeds, however, the frequency of the triggering pulses is increased and current is connected, only shortly before the ignition time, and the ignition impulse to the coil is weak so that ignition degrades with increased engine speed. In addition, some prior art electronic ignition systems have provided a constant of time for the coil current causing an undesirable power dissipation at low speeds, or low rpms of operation of the engine- This is highly undesirable and creates a heavy drain on the battery of the vehicle in which the ignition system is used.

It is desirable to employ an ignition system which does not waste power at low rpms, and which employs a constant ratio of on time (charging current through the primary winding of the coil) to off time, or a constant dwell irrespective of the speed of the engine in order to improve the performance of transistor ignition systems over those previously known.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved electronic ignition system for an internal combustion engine.

It is another object of this invention to provide an electronic ignition system which operates with a constant dwell time of the charging current through the primary winding of the ignition coil.

It is a further object of this invention to provide an ignition circuit of the constant dwell type at engine speeds above a predetermined amount, and as a constant off type at engine speeds below such predetermined amount.

In accordance with a preferred embodiment of this invention, a first transistor switch is rendered conductive and nonconductive by input pulses obtained from a magnetic pickup which can be located in a conventional distributor housing. This first transistor switch is connected to a source of potential through a first current source. A second transistor switching device has the emitter collector electrode path connected to the source of potential, and the base of the second transistor switching device is connected to the potential source through a second current source. A charge storage capacitor is connected between the collector of the first transistor switching device and the base of the second transistor switching device, with a charge path for the capacitor being completed, when the first transistor switching device is nonconductive from the first current source through the base emitter path of the second transistor, which is rendered conductive. The primary winding of the coil is coupled through a semiconductor switch with the collector of the second transistor switching device so that current flows through the primary winding when the second transistor is conductive. When the first transistor is subsequently rendered conductive, the second transistor is immediately biased to a non-conductive state, and the capacitor initially discharges and then recharges through the second current source and the collector/emitter path of the first transistor. When the capacitor becomes charged in the reverse direction by the second current source, it biases the second transistor back to its state of conduction and coil current commences to flow, following the interruption of coil current which occurred during the time that the second transistor was non-conductive. By adjusting the ratios of current flow in the two current sources, the percent of time that current flows through the coil can be adjusted to a predetermined amount to provide constant dwell.

BRIEF DESCRIPTION OF THE DRAWINGS The sole FIGURE of the drawing is a detailed schematic circuit diagram of a preferred embodiment of the invention.

DETAILED DESCRIPTION Referring now to the drawing, there is shown a transistor ignition system for providing a constant dwell (constant ratio of on time to off time of current flow in the primary winding of the ignition coil of an automobile) ignition system.

Direct current voltage for providing the current and voltage supplies for operating the system shown in the drawing is obtained from a battery which may be coupled to a voltage supply terminal 10. The battery is connected through an ignition switch 11 and a ballast resistor 12 to the upper end of the primary winding of an ignition coil 14, which may be of any suitable type. The lower end of the primary winding of the ignition coil 14 is connected through the collector/emitter path of a switching transistor 15, with the emitter of the switching transistor 15 being connected to ground. The operation of the circuit is such that when the transistor 15 is non-conductive no current is permitted to flow through the primary winding of the coil 14, and when the transistor 15 is rendered conductive current flows through the primary winding of the coil 14 to charge the coil. As a consequence, each time the transistor 15 is rendered non-conductive the flux in the coil 14 collapses to induce the spark voltage in the secondary winding of the coil 14, which is connected to the spark plugs (not shown) of the engine.

The circuit shown in the drawing operates to provide a constant dwell, or constant ratio of on time to off time for the current flow in the primary winding of the coil 14. This is accomplished by the control circuit which drives the transistor switch 15.

Preferably, a magnetic pickup device (not shown) is positioned within the distributor of a vehicle to produce a generally sinusoidal output waveform 17, which is applied to the ignition system across a pair of input terminals 19 and 20. A suitable pickup device which can be used to produce the signal 17 may be of the type disclosed in US. Pat. No. 3,390,668, issued to Arthur G. Hufton and assigned to the same assignee as the present application. The input pulses are applied across a capacitor 21 and through a diode 22 and resistor 24 to the base of a first switching transistor (NPN) 26, the emitter of which is connected to ground, and the collector of which is connected through a resistor 27 and diode 28, to a voltage supply lead 30 which is coupled through a resistor 31 to the ignition switch 11. The resistor 27 operates as a current source for the circuit. Assume initially that the transistor 26 is rendered nonconductive. This condition occurs on the negative half cycles of the input waveform 17, which causes the emitter base junction of the transistor 26 to be reverse biased. When this occurs, the current supplied by the current source 27 flows through a charge storage capacitor 34 and the base emitter junction of another switching transistor 36 to charge the capacitor 34 from the current source resistor 27. At'this time the transistor 36 is rendered conductive causing a relatively low, or near ground potential to appear on its collector. This potential applied to the base of an amplifier transistor 38 through a coupling resistor 39 renders the transistor 38 non-conductive. The collector of the transistor 38 is connected to the voltage supply lead 30 through a diode 41 and a resistor 40, so that the junction of the diode 41 and resistor 40 rises to a voltage nearer the voltage on the supply lead 30. This, in turn, renders an NPN Darlington transistor 42 conductive causing a relatively positive potential to appear across a load resistor 43 biasing the switching transistor into a state of conduction. Thus, current flows through the primary winding of the ignition coil 14.

when the positive half cycle of the waveform l7 obtained from the magnetic pickup occurs, the reverse bias is removed from the base emitter junction of the transistor 26, and it is rendered conductive and held in conduction by the potential applied to its base through resistor 45 and 24 and resistors 49 and 52. When the transistor 26 initially conducts, the potential applied by the discharge of capacitor 34 to the base of the transistor 36 renders the transistor 36 non-conductive, and the capacitor 34 commences charging in the opposite direction through a second current source resistor 47 and the collector/emitter path of the switching transistor 26. When the transistor 36 is rendered nonconductive, the conductive states of transitors 38, 42 and 15 change so that the transistor 15 now also is nonconductive and current flow through the primary winding of the ignition coil 14 is interrupted. This causes a rapid collapse of flux in the primary winding of the ignition coil 14, inducing the high voltage spark pulse in the secondary winding thereof.

The transistor 36 remains non-conductive until the charge on the capacitor 34 reaches the point where the base emitter junction of the transistor 36 once again is forward biased. At this time the transistor 36 commences conduction and the conduction holding potential through the resistor 52 to the base of the transistor 26 is removed. Once again the transistors 38, 42 and 15 attain the original state, which has been described, with the transistor 15 being rendered conductive to start the flow of current through the primary winding ofthe ignition coil 14. It should be noted that depending upon the ratio selected between the currents supplied by the current source resistors 27 and 47, the time in the positive half cycle of the waveform 17 at which the transistor 36 is rendered conductive can be varied. Or stated in another way, by selecting the value of resistors 27 and 47 the time base for charging the capacitor 34 through each of these resistors can be controlled. For engine operating speeds above a predetermined amount, the size of the capacitor 34 is selected so that it does not become fully charged by the current from the current source 27 during the negative half cycle of the waveform 17. The current of the current source resistor 47 is selected to be greater than the current supplied by the current source resistor 27 so that discharge of the capacitor takes place in less than a half cycle of the waveform 17, and the selection of the value of the resistors 27 and 47 to establish the ratios of the currents supplied determines the percentage of time that the transistor 36 is turned off for each complete cycle of the waveform 17. At higher speeds of operation the charging time on the negative half cycles of the waveform 17 of the capacitor 34 is less than at lower speeds so that the charge reached by the capacitor 34 prior to the time that the transistor 26 is rendered conductive is less for high speed operation than for low speed operation. Thus, it takes less time to recharge the capacitor in the opposite direction for high speed operation than for low speed operation, and the constant dwell or constant percentage of on time to off time of the circuit is preserved irrespective of the speed of operation of the engine, which in turn is reflected by the frequency of the waveform 17.

It should be noted that for whatever portion of the half cycle (positive) of the waveform 17 that both the transistors 36 and 26 are conductive no further charging in either direction takes place. This establishes the initial or threshold condition for charging the capacitor in the initial direction when the transistor 26 once again is rendered non-conductive on the negative half cycle of the waveform 17. At engine speeds below idling rpm, engine acceleration greatly reduces the time between trigger pulses and constant dwell at such low speeds is not desirable. The circuit automatically reverts to a constant off time circuit at low rpms, since the capacitor 34 is permitted to charge up to the full available voltage is less than one-half cycle of the input waveform applied to the transistor 26 for such low rpm operation. This then establishes the maximum length of time which is required for recharging the capacitor 34 in the opposite direction to once again render the transistor 36 conductive. This lower threshold at which the circuit switches from a constant dwell type of operation to a constant off operation may be selected to be at any engine speed desired, and a preferable speed has been determined to be below 400 rpm so that this is in the starting rpm range only.

To maintain stable operating voltages in the circuit, and further to provide voltage protection for the low power dwell circuitry, a Zener diode 50 is connected between the voltage supply lead 30 and ground. Another Zener diode 51 is coupled between ground and a resistor 53 connected to the ignition switch 11 and from which collector potentials for the Darlington driver transistor 42 is obtained. The Zener diode 51 provides protection for the drive circuit of the Darlington transistor 42 from transients generated in the load. A pair of series connected Zener diodes, 56 and 57. are coupled between the lower ends of the primary winding of the coil 14 and ground provides protection for the.

output transistor from high voltage transients produced during the collapse of flux of the coil 14. The diode 61 provides protection for the Zener diode 60 from high forward currents during starting with a reverse battery protection.

Since the ballast resistor 12 presents an undesirable high impedance during starting, there is shown a provision including a start switch 65 and a relay 66, 67 for shunting the ballast resistor during operation of the starter 69. During the starting cycle drive current for the Darlington amplifier 42 is provided through a diode 70 from the starter switch 65 when the ignition key is in the start position. This provides a low resistance path for low voltage operation, and allows additional resistance to be insertedin the drive circuit to reduce drive power dissipation during higher voltage normal run conditions of the circuit. During normal run conditions the operating current supplied to the collectors of the Darlington transistor 42 is supplied through the resistor 53 when the start switch 55 is opened. A resistor 72 provides a load on the starter relay coil 66 to reduce voltage stress on the diode 70 during the starting operation.

In one working embodiment of the invention the following values for the listed circuit components were used to provide a constant dwell on or charging time of the primary winding of the ignition coil 14 to of time of 70 percent. The listing of these values is for information only and is not meant to limit the scope of the invention in any manner.

1. Resistor 24 6.8K ohms 2. Resistor 45 15K ohms 3. Resistor 27 330K ohms 4. Resistor 47 240K ohms 5. Capacitor 34 22uf 6. Zener Diode 50 10 volts I claim:

1. An electronic ignition system for alternately charging and collapsing the field in an ignition coil to produce a spark for firing an internal combustion engine, including in combination, first switching circuit means having an input and an output and operable between an on and an off condition, with the output thereof coupled with the ignition coil for alternately charging and collapsing the field in the ignition coil, second switching circuit means having an input and an output and operable between an on and an off condition, control means connected to the input of said second switching circuit means and controlling the same between the on and off condition in direct relation to engine RPM, a capacitor having a first terminal connected with the output of said second switching circuit means and a second terminal connected with the input of said first switching circuit means, a first direct current source connected to one of said first and second terminals of said capacitor, a second direct current source connected to the other of said first and second terminals of said capacitor, said second switching circuit means being operated to one of said on and off conditions by said control means to charge said capacitor through said first direct current source to operate said first switching circuit means to one of said on and off conditions to charge the ignition coil, said second switching circuit means being further operated to the other one of said on and off conditions by said control means to discharge said capacitor through said second direct current source to operate said first switching circuit means to the other one of said on and off conditions to collapse the field in the ignition coil to cause an ignition spark, resistive means coupled from said first switching circuit means to said second switching circuit means to hold said second switching circuit means in said other one of the on and off conditions thereof with said first switching circuit means in said other one of the on and off conditions thereof, and the ratio of current passing through said first direct current source to the current passing through said second direct current source at engine rpm above cranking speed is such that said capacitor charges to an amount less than the total capacity thereof prior to the discharge of the same thereby providing charging of the ignition coil for a constant percent of the total period'between ignition sparks, whereas at engine cranking speed, the frequency of operation of said second switching circuit means permits said capacitor to completely charge through said first direct current source to provide a constant off time of the current through the ignition coil at such cranking speed.

2. The electronic ignition system of claim 1 wherein said first direct current source includes first resistor means connected between a direct current potential and the first terminal of said capacitor, and said second direct current source includes second resistor means connected between a direct current potential and the second terminal of said capacitor, said first resistor means being greater in value than said second resistor means. I

3. An electronic ignition system for charging and discharging an ignition coil to produce a spark to operate an internal combustion engine including in combination:

circuit means for conducting direct current through the ignition coil to charge the same; a first semiconductor switching device having first,

second and control electrodes; first resistor means connected between a direct current potential and the first electrode of said first semiconductor switching device; pulsing means for producing pulses of first and second polarities at a frequency proportional to engine RPM coupled with the control electrode of said first semiconductor switching device for operating said second semiconductor switching device between an on and an off condition in response to respective pulses of opposite polarity from said pulsing means;

second semiconductor switching device having first, second and control electrodes and operable between an on and an off condition, with the first electrode thereof coupled with said circuit means for controlling the flow of current through the ignition coil; second resistor means having a value of resistance less than that of said first resistance means connected between a direct current potential and the control electrode of said second semiconductor switching device;

means connecting a direct current potential to the first electrode of said second semiconductor switching device;

charge storage means connected between the first whereby with said first semiconductor switching device biased to the off condition by said pulsing means, said charge storage means charges through said first resistor means to bias said second semiconductor switching device to the on condition to control said circuit means to charge the ignition coil, and with said first semiconductor switching device biased to the on condition by said pulsing means, said charge storage means discharges through said second resistor means to control said circuit means to produce an ignition spark, the resistance values of said first and second resistor means and the charge storage value of said charge storage means being selected such that at cranking speed of the engine, the low frequency of switching said first semiconductor switching device by said pulsing means permits said charge storage device to charge completely between each spark pulse, thereby providing a constant off time of the current through the ignition coil; and

said resistance values of said first and second resistor means and the charge storage capacity of said charge storage means further being selected such that at engine RPM speeds above cranking speed, said charge storage means does not charge to full value and is discharged completely during each ignition spark produced by the coil; thereby insuring that the charging current through the coil is maintained in the on condition for a constant percent of the total period between spark pulses for such engine speeds above cranking speed.

4. The combination according to claim 3 wherein said charge storage means is a capacitor and said first and second semiconductor switching devices comprise first and second transistors respectively, the collectors of which correspond to the first electrodes, the emitters of which correspond to the second electrodes and the bases of which correspond to the control electrodes, with the emitter electrodes of both of said first and second transistors being connected to a point of reference 

1. An electronic ignition system for alternately charging and collapsing the field in an ignition coil to produce a spark for firing an internal combustion engine, including in combination, first switching circuit means having an input and an output and operable between an on and an off condition, with the output thereof coupled with the ignition coil for alternately charging and collapsing the field in the ignition coil, second switching circuit means having an input and an output and operable between an on and an off condition, control means connected to the input of said second switching circuit means and controlling the same between the on and off condition in direct relation to engine RPM, a capacitor having a first terminal connected with the output of said second switching circuit means and a second terminal connected with the input of said first switching circuit means, a first direct current source connected to one of said first and second terminals of said capacitor, a second direct current source connected to the other of said first and second terminals of said capacitor, said second switching circuit means being operated to one of said on and off conditions by said control means to charge said capacitor through said first direct current source to operate said first switching circuit means to one of said on and off conditions to charge the ignition coil, said second switching circuit means being further operated to the other one of said on and off conditions by said control means to discharge said capacitor through said second direct current source to operate said first switching circuit means to the other one of said on and off conditions to collapse the field in the ignition coil to cause an ignition spark, resistive means coupled from said first switching circuit means to said second switching circuit means to hold said second switching circuit means in said other one of the on and off conditions thereof with said first switching circuit means in said other one of the on and off conditions thereof, and the ratio of current passing through said first direct current source to the current passing through said second direct current source at engine rpm above cranking speed is such that said capacitor charges to an amount less than the total capacity thereof prior to the discharge of the same thereby providing charging of the ignition coil for a constant percent of the total period between ignition sparks, whereas at engine cranking speed, the frequency of operation of said second switching circuit means permits said capacitor to completely charge through said first direct current source to provide a constant off time of the current through the ignition coil at such cranking speed.
 2. The electronic ignition system of claim 1 wherein said first direct current sourcE includes first resistor means connected between a direct current potential and the first terminal of said capacitor, and said second direct current source includes second resistor means connected between a direct current potential and the second terminal of said capacitor, said first resistor means being greater in value than said second resistor means.
 3. An electronic ignition system for charging and discharging an ignition coil to produce a spark to operate an internal combustion engine including in combination: circuit means for conducting direct current through the ignition coil to charge the same; a first semiconductor switching device having first, second and control electrodes; first resistor means connected between a direct current potential and the first electrode of said first semiconductor switching device; pulsing means for producing pulses of first and second polarities at a frequency proportional to engine RPM coupled with the control electrode of said first semiconductor switching device for operating said second semiconductor switching device between an on and an off condition in response to respective pulses of opposite polarity from said pulsing means; a second semiconductor switching device having first, second and control electrodes and operable between an on and an off condition, with the first electrode thereof coupled with said circuit means for controlling the flow of current through the ignition coil; second resistor means having a value of resistance less than that of said first resistance means connected between a direct current potential and the control electrode of said second semiconductor switching device; means connecting a direct current potential to the first electrode of said second semiconductor switching device; charge storage means connected between the first electrode of said first semiconductor switching device and the connection of said second resistor means and the control electrode of said second semiconductor switching device; whereby with said first semiconductor switching device biased to the off condition by said pulsing means, said charge storage means charges through said first resistor means to bias said second semiconductor switching device to the on condition to control said circuit means to charge the ignition coil, and with said first semiconductor switching device biased to the on condition by said pulsing means, said charge storage means discharges through said second resistor means to control said circuit means to produce an ignition spark, the resistance values of said first and second resistor means and the charge storage value of said charge storage means being selected such that at cranking speed of the engine, the low frequency of switching said first semiconductor switching device by said pulsing means permits said charge storage device to charge completely between each spark pulse, thereby providing a constant off time of the current through the ignition coil; and said resistance values of said first and second resistor means and the charge storage capacity of said charge storage means further being selected such that at engine RPM speeds above cranking speed, said charge storage means does not charge to full value and is discharged completely during each ignition spark produced by the coil; thereby insuring that the charging current through the coil is maintained in the on condition for a constant percent of the total period between spark pulses for such engine speeds above cranking speed.
 4. The combination according to claim 3 wherein said charge storage means is a capacitor and said first and second semiconductor switching devices comprise first and second transistors respectively, the collectors of which correspond to the first electrodes, the emitters of which correspond to the second electrodes and the bases of which correspond to the control electrodes, with the emitter electrodes of both of said first and second transistors being connected tO a point of reference potential. 